The present invention relates generally to the inspection of components employing eddy current techniques and, more particularly, to the processing of signals from an eddy current probe array which is mechanically scanned in one direction.
Eddy current inspection is a commonly used technique for non-destructively detecting discontinuities or flaws in the surface of various components, for example aircraft engine parts. Very briefly, eddy current inspection is based on the principle of electromagnetic induction in which a drive coil is employed to induce eddy currents within the material under inspection, and secondary magnetic fields resulting from the eddy currents are detected by a sense coil, generating signals which are subsequently processed for the purpose of detecting flaws.
Eddy current testing for flaws in conductive materials is typically done by mechanically scanning a single probe in two dimensions. For example, patent application Ser. No. 07/772,761, filed Sep. 16, 1991, now U.S. Pat. No. 5,345,514, entitled "An Improved Method for Inspecting Components Having Complex Geometric Shapes" describes methods for interpreting eddy current image data acquired by a single probe, particularly in the context of inspecting a high pressure turbine (HPT) disk dovetail slot.
Eddy current images as created by scanning a surface with a single probe are usually highly over-sampled because the probe is moved in incremental steps that are just a small fraction of the dimension of the active area of the probe. Therefore, as the probe moves across a defect, numerous image data points "detect" the defect, i.e. they have amplitude values that correspond to defect levels. This over-sampling occurs in two dimensions, so an image of a point source (e.g. a small hole) actually appears as the active area of the probe, rather than just the point it represents. Because of this, image processing routines that take advantage of the highly correlated image information can be used to improve the likelihood of detecting small defects in eddy current images. The above-identified application Ser. No. 07/772,761 discloses one such approach to image processing.
Although effective, the single probe scanning method is time consuming. Probe arrays have been developed to improve the scanning rate, as well as to increase flaw detection sensitivity. Probe arrays include several identical elements that are scanned simultaneously in parallel. Eddy current arrays and array systems have been described in the literature, for example Bert A. Auld, "Probe-Flaw Interactions with Eddy Current Array Probes", Review of Progress in Quantitative NDE 10, edited by D. O. Thompson and D. E. Chimenti (Plenum Press, New York, 1991), pages 951-955; A. J. Bahr, "Electromagnetic Sensor Arrays--Experimental Studies", Review of Progress in Quantitative NDE 10, edited by D. O. Thompson and D. E. Chimenti (Plenum Press, New York, 1991) pages 691-698; Yehuda D. Krampfner and Duane D. Johnson, "Flexible Substrate Eddy Current Coil Arrays", Review of Progress in Quantitative NDE 7, edited by D. O. Thompson and D. E. Chimenti (Plenum Press, New York, 1988), pages 471-478; and Mirek Macecek, "Advanced Eddy Current Array Defect Imaging", Review of Progress in Quantitative NDE 10, edited by D. O. Thompson and D. E. Chimenti (Plenum Press, New York, 1991), pages 995-1002.
As another example, General Electric High Density Interconnected (HDI) technology has been used to fabricate flexible eddy current probe arrays. In particular, Hedengren et al. application Ser. No. 07/696,455, filed May 6, 1991, the entire disclosure of which is hereby expressly incorporated by reference, discloses an eddy current probe array comprising a plurality of spatially correlated eddy current probe elements disposed within a flexible interconnecting structure which may be employed to collect a discrete plurality of spatially correlated eddy current measurements for non-destructive near surface flaw detection. An array of such elements can, in a single unidirectional scan, accommodate inspecting an area covered by the active width of the array. This array structure can flexibly conform to accommodate inspection of large, irregular, curved conductive surfaces which cannot readily be inspected by conventional means.
Although eddy current probe arrays have a speed advantage, in contrast with single probe scanning there is no over-sampling along the common axis of the elements; only a single measurements can be acquired across the width of one element, and a small defect may be detected only by one element. Therefore, image processing as applied to images created by single probe scanning cannot be used as effectively for array data. Instead, signal processing may be applied to individual data traces acquired for each element in a direction perpendicular to a row of elements.
Eddy current data from parts with irregular geometries is particularly difficult to process and analyze. One challenging problem arises from the fact that the probe moves from air past a first edge of a part being inspected, over the surface of the part and then over a second edge of the part back into air. This gives rise to potentially large signals at the edges. Any signals due to cracks at the edges must be detected even though they are buried in the large edge signals.
Although there are known methods for emphasizing flaw signals, such as the background subtraction method, difficulties arise when applied to interpreting data from eddy current probe arrays. For example, alignment between test waveforms and reference waveforms is critical, and yet a mechanical scanner is unable to return to an identical initial position to originate data collection time after time. Moreover, various factors such as "lift off" introduce DC level variations.